Methotrexate (MTX) may be the single most important drug in the treatment of children with ALL. It has not been replaced in 4 decades. During this time, a great deal of research has revealed the mechanisms for tumor resistance (more applicable to models in vitro) and the spectrum of anti- folate toxicity. Studies in the last few years have shown an association between lymphoblast metabolism of MTX and long-term survival. A main goal of our laboratory is to increase the therapeutic efficacy of anti-folates. This will be done by obtaining more information about tumor and host metabolism. Using functional assays (metabolism of radio-labeled drug) as well as immunoblotting and nucleic acid probes, we are searching for differences in patients and tumor cells in order to identify patients who are likely to be cured, to be poor responders, and/or those who may experience excessive toxicity. Our studies with Aminopterin (AMT) in isolated leukemia blasts as well as established cell lines have confirmed AMT's greater metabolism and potency in vitro. The goal of this application is to utilize our technical capabilities to test the hypothesis that AMT is at times the better drug. Phase I, bioavailability and phase II/III trials of AMT will be done. With the additional knowledge and monitoring capabilities that we have gained in the nearly 50 years since its initial use, we are better equipped to prevent, predict, and ameliorate AMT toxicities.